1. Field of the Invention
The present invention relates to melt-processable polyimide and more particularly relates to polyimide having excellent processability and a process for preparing the polyimide.
2. Related Art of the Invention
Conventionally, polyimide prepared by reaction of tetracarboxylic acid dianhydride with diamine is excellent in mechanical strength and dimensional stability in addition to exhibiting high heat-resistance and also has flame retardance and electrical insulative properties. Consequently, polyimide has been used in the fields such as electric and electronic devices, space and aeronautic equipment, and transport machinery, and is expected for wide use in additional fields where heat resistance is required.
Various kinds of polyimide which exhibit excellent properties have conventionally been developed. The polyimide, however, has no distinct glass transition temperature, though excellent in heat resistance, and must be processed by such means as sinter molding when used as a molding material. Other polyimides dissolve in halogenated hydrocarbon solvents, though excellent in processability, and lead to problems in solvent resistance. Thus, both merits and drawbacks have simultaneously been found in the properties of conventional polyimide. For example, polyimides prepared from 4,4'-diaminodiphenyl ether and pyromellitic dianhydride has a fundamental skeleton of the formula (V): ##STR3## and has been known as high heat-resistant polyimide Kapton and Vespel (Trade mark; E. I. Du Pont de Nemours & Co.). The polyimide, however, has no distinct glass transition temperature and is difficult to process in molding articles, though excellent in heat resistance. Thus, the polyimide has a disadvantage that a technique such as sinter molding is required for processing.
Researches have been carried out on the use of diamine having an isopropylidene radical as a diamine component of polyimide. For example, polyimide which is prepared from pyromellitic dianhydride and has a fundamental skeleton of the formula (VI): ##STR4## has been reported by G. M. Bower et al in J. Polymer Sci., pt-A, 1, 3135.about.3150(1963) and by C. E. Srog et al. in J. Polymer Sci., pt-A, 3, 1373.about.1390(1965). The polyimide, however, does not exhibit thermoplasticity and has no melt-flowability. As a result, the polyimide is disadvantageous in difficulty of processing.